A previous back surface electrode-type solar cell is schematically shown in FIG. 14 as a cross sectional view. The back surface electrode-type solar cell 210 produced by a previous art will be described by referring to FIG. 14. On the light-receiving surface side of an N-type silicon substrate 213, a rugged shape 214 and an FSF (Front Surface Field) layer 215, which is an N-type diffusion layer, are formed. On the rugged shape 214, a dielectric passivation layer (a surface passivation layer) 217 which contains silicon dioxide and an antireflective film 216 which contains silicon nitride are formed from the side of the N-type diffusion layer 213.
On the backside of the N-type silicon substrate 213, N-type doped N-type diffusion layers 220 and P-type doped P-type diffusion layers 221 are alternately formed. In addition, on the backside of the N-type silicon substrate 213, an oxide layer (the first backside passivation film) 219 is formed. On the N-type diffusion layer 220, an N-type contact electrode 211 is formed; and on the P-type diffusion layer 221, a P-type contact electrode 212 is formed. These contact electrodes, which are joined directly to the substrate itself, can also function as finger electrodes for collecting current.
FIG. 15 is a top view schematically showing the appearance of the backside of the previous back surface electrode-type solar cell. As shown in FIG. 15, the back surface electrode-type solar cell is provided with a pair of bus bar electrodes (an N-type bus bar electrode 222, a P-type bus bar electrode 223) at the edge of the substrate for collecting current from finger electrodes (the N-type contact electrode 211, the P-type contact electrode 212). Although the electrodes nearest to the periphery of the substrate are depicted as N-type contact electrodes in FIG. 15, they may be P-type contact electrodes or metal electrodes of different type with each being P-type and N-type.
To improve the efficiency of the back surface electrode-type solar cell, full enlargement of the P-type diffusion layer, which is a power generation layer, can be expected to increase short circuit current. Accordingly, it is desirable to form the region of the P-type diffusion layer widely such as the area proportion of the P-type diffusion layer and the N-type diffusion layer in a range of 80:20 to 90:10. When the area of contact between the substrate and the contact electrodes (hereinafter, also referred to as contact area) is decreased as possible, and the passivation regions are enlarged, increase of open circuit voltage can be expected. Accordingly, it is desirable to design the contact region as small as possible by making the contact electrodes in thin line shapes or dot shapes.
Patent Literature 1 discloses a back surface electrode-type solar cell in which the contact area of the electrodes and the substrate is suppressed to the lowest possible, and the passivation regions are enlarged by three steps of forming contact electrodes, covering the portion other than the contact electrodes with an insulator film, and forming a wiring electrode.
FIG. 17 is a top view schematically showing the appearance of the backside of the previous back surface electrode-type solar cell disclosed in Patent Literature 1. In the solar cell of Patent Literature 1, however, only one pair of bus bar electrodes (an N-type bus bar electrode 222, a P-type bus bar electrode 223) are formed at the periphery of the substrate (see FIG. 17). In this arrangement, the finger electrodes are long, and accordingly the wiring resistance becomes extremely large, which causes lowering of a fill factor. This wiring resistance becomes larger in proportion to the length of wiring. It is considered that this can be solved by designing the wiring electrodes (finger electrodes) to have enlarged cross-section or the finger to have shortened length.